A device for the heat treatment of powdery substances by means of a high-temperature plasma

ABSTRACT

Apparatus for use therein for heat treating irregular shaped particles of nuclear fuels so as to produce granules of substantially spherical shape comprising preheating the irregular nuclear fuel particles to high temperatures, but below the melting point thereof, separate preheating a carrier gas for the particles, admixing the heated particles with the heated gas, introducing the mixture in predetermined amounts into a high temperature plasma zone, heating the particles in said zone to effect the melting of the surfaces of the particles and to thereby produce the spherical granules.

n SH Inventors Peter Weimar Kolberger Str. 28A, Karlsruhe; HarryLlepelt, Bruhl Str. 27, Stal'fort, both of, Germany Appl. No. 560,469Filed June 27, 1966 Patented Aug. 10, 1971 Priority June 28, 1965Germany A DEVICE FOR THE HEAT TREATMENT OF POWDERY SUBSTANCES BY MEANSOF A HIGH- TEMPERATURE PLASMA 10 Claims, 1 Drawing Fig.

US. Cl 219/76, 219/121 Int. Cl 823k 9/04 Field ofSearch.... 217/69,

[56] References Cited UNITED STATES PATENTS 3,182,361 5/1965 TrimblePrimary Examiner-Joseph V. Truhe Assistant ExaminerC L. AlbrittonAttorney-Burgess, Dinklage and Sprung ABSTRACT: Apparatus for usetherein for heat treating irregular shaped particles of nuclear fuels soas to produce granules of substantially spherical shape comprisingpreheating the irregular nuclear fuel particles to high temperatures,but below the melting point thereof, separate preheating a carrier gasfor the particles, admixing the heated particles with the heated gas,introducing the mixture in predetermined amounts into a high temperatureplasma zone, heating the partides in said zone to effect the melting ofthe surfaces of the particles and to thereby produce the sphericalgranules.

PATENIEU Aumlsn l'llllll A DEVICE FOR THE HEAT TREATMENT OF POWDERYSUBSTANCES BY MEANS OF A HIGH-TEMPERATURE PLASMA The invention relatesto a method and a device for the heat treatment of powdery substances bymeans of a high-temperature plasma, especially for melting the grains ofpowdery high melting point substances for the purpose of converting theangular into a spherical grain shape.

In the fabrication of ceramic nuclear reactor fuel elements it isintended to achieve as high a density of the fuel as possible. Now, thishas turned out to be possible when granular huclear fuel consists ofspherical grains. For this-purpose the angular grains of a powder areput through a high-temperature plasma causing the grain material to meltand the substance to reemerge from the plasma in a spherical shape. Thedevices best suited for this purpose have been the so-calleddirect-current plasma burners where current are discharge is generatedbetween a rod-type cathode and an anode concentrically surrounding it.The gas which is to form the plasma is fed through this concentricdischarge channel, and the powder is introduced into the generatedplasma by a carrier gas.

It has been shown that in devices of this type the percentage of energyof the plasma which is transferred to the powder is relatively small andthat hence a longer period of sojourn in the plasma up to the meltingprocess is necessary. However, this would require relatively largestructures to generate a plasma beam safeguarding this period ofsojourn.

It is the object of this invention to provide a method and a device inwhich the period of sojourn of powdery substances in a high-temperatureplasma for the purpose of melting is as long as possible and where theplasma apparatus can be kept relatively small so that there will be acorrespondingly favorable'heat utilization and an optimum technicalefficiency.

Now, it was found out that this object is obtained by the methodaccording to the invention for the heat treatment of powdery substances,especially for melting the grains of highmelting point substances bymeans of a high-temperature plasma, by the fact that the powderysubstance before being brought into the plasma region is heated tohigher temperatures, preferably above 1 ,000 C. (preheated). Inparticular it is an advantage in a process, in which the powderysubstance is finely dispersed .in a carrier gas and transported into theplasma region by this gas, to preheat the powdery substance as well asthe carrier gas. it has proved to be useful to preheat the powderysubstance and the carrier gas separately and mix the two afterwards. Inthis way the powdery substance is dried and, at the same time, only thedried substance free from all adsorbed gases is introduced into thecarrier gas. For this purpose it is useful to preheat the powderysubstance and the carrier gas in a common heating device. Moreover, ithas proved to be very advantageous to generate the plasma out of thecarrier gas containing an admixture of the powdery substance by means ofan arc discharge, a high frequency discharge or the like.

A device for the execution of the method according to the invention witha system generating a high-temperature plasma (plasma-burner) accordingto the invention comprises a preheating device for preheating thepowdery substance which is connected in series with the plasma burnerand, moreover, a preheating device for the carrier gas connected inseries with the plasma burner. It is useful to combine the preheatingdevice for the powdery substance and that for the carrier gas into onecommon preheating device. At the outlet of the preheating deviceaccording to the invention there is a mixing nozzle for the introductionof the powdery substance into the carrier gas, to control the flow ofthe powdery substance through the opening of the mixing nozzle there maybe a vibrating device periodically changing the opening resistance. Thedosage of the powdery substance can be controlled in a very simple wayby a vibrating device with adjustable vibrating frequency and amplitude.

If a light are plasma burner consisting of a pin-type cathode andhollow-cylinder anode is used, it is particularly advantageous to designthe cathode according to the invention at the same time at leastpartially as the inlet line for the carrier gas transporting the powderysubstance. Now, in order to avoid cooling off of the gas as soon as itleaves the preheating device and enters the cathode designed as an inletline, the cathode is designed as a resistance heating element to beheated by the current flowing to the light are. The support of thecathode may be cooled by a liquid. Moreover, it is useful to surroundthe cathode by a heat insulating sleeve, e.g. of a highly refractoryceramic material, forming the flow channel for the carrier gas in therange in which the anode is surrounded by the ca'rrier gas transportingthe powdery substance.

A special way of execution of the device according to the invention isshown in the FIGURE.

The powdery substance 1 is contained in a tubular container 2 andsurrounded by a helical pipe 3 containing the carrier gas flow. Thetubular container 2 as well as the pipe 3 surrounding it are housed in afurnace 4 acting as a preheating device. At the end of the tubularcontainer 2 there is a nozzle 5 (mixing nozzle) through which thepowdery substance 1 is dispersed in the carrier gas and from which bothflow through a common line 6 to the cathode 7. The mixing nozzle 5consists ofa funnel-shape'd nozzle opening which holds a nozzle needle 8which is caused to vibrate in its longitudinal direction by a vibratingdevice 9. The cathode 7 in its bottom section is designed as an inletline of the carrier gas containing the powdery substance and connectedwith the negative pole of a direct-current source by the contact piece10. The upper section 11 of the cathode is designed as a pin and issurrounded by the flow of the carrier gas with the powdery substance.This cathode pin 11 forms a flow channel for the carrier gas togetherwith the concentric anode 12. In order to prevent the carrier gas andthe powdery substance it contains from transferring their heat to thewalls of the water-cooled anode 12 or the other surrounding walls, aheat insulating cylinder 13 made of ceramic material is provided. Theanode 12 as well as the cathode support 14 are cooled by a liquid. Theanode 12 is connected with the positive pole of the direct-currentsource by the contact piece 15. The high-current arc maintained by thedirect-current source and generated at point 16 between cathode 7 andanode 12 forms the plasma by means of the carrier gas and the powderysubstances contained in it. The electrical resistance of cathode 7 is sodimensioned that owing to the current flow through it simultaneouslyacts as a resistance heating element to preheat the carrier gas. A

We claim:

1. Apparatus for heat treating irregular shaped particles of nuclearfuels for producing granules of substantially spherical shape comprisingmeans for preheating the irregular nuclear fuel particles, means forpreheating a carrier gas for transporting said particles, means formixing said preheated particles and carrier gas, high temperature plasmaburner means connected in series with said means for preheating saidparticles and said means for preheating said carrier gas and means forintroducing said mixture of particles and gas in predetermined amountsto said high temperature plasma burner means whereby the surfaces ofsaid particles are melted producing the desired spherical shaping.

2. Apparatus according to claim 1 wherein said means for preheating saidparticles and for preheating said carrier gas are combined into onecommon preheating means.

3. Apparatus according to claim 2 including a mixing nozzle forintroducing said particles into the carrier gas, said mixing nozzlebeing arranged at the outlet of said preheating means.

4. Apparatus according to claim 3 including vibrating means adapted forperiodically changing the opening resistance for controlling the flow ofparticles through the opening of said mixing nozzle.

5. Apparatus according to claim 4 wherein said vibrating means has anadjustable vibrating frequency and amplitude.

6. Apparatus according to claim 3 wherein said plasma burner meanscomprise a light arc plasma burner including a pin-type cathode and ahollow-cylindrical anode concentrically surrounding said cathode whereinsaid cathode is constructed so as to serve at least partially as aninlet conduit for the carrier gas transporting said particles.

7. Apparatus according to claim 6 wherein said cathode is constructed asa resistance heating element adapted to be heated by current flowing tosaid light are.

8. Apparatus according to claim 6 including a support for

2. Apparatus according to claim 1 wherein said means for preheating saidparticles and for preheating said carrier gas are combined into onecommon preheating means.
 3. Apparatus according to claim 2 including amixing nozzle for introducing said particles into the carrier gas, saidmixing nozzle being arranged at the outlet of said preheating means. 4.Apparatus according to claim 3 including vibrating means adapted forperiodically changing the opening resistance for controlling the flow ofparticles through the opening of said mixing nozzle.
 5. Apparatusaccording to claim 4 wherein said vibrating means has an adjustablevibrating frequency and amplitude.
 6. Apparatus according to claim 3wherein said plasma burner means comprise a light arc plasma burnerincluding a pin-type cathode and a hollow-cylindrical anodeconcentrically surrounding said cathode wherein said cathode isconstructed so as to serve at least partially as an inlet conduit forthe carrier gas transporting said particles.
 7. Apparatus according toclaim 6 wherein said cathode is constructed as a resistance heatingelement adapted to be heated by current flowing to said light arc. 8.Apparatus according to claim 6 including a support for said cathode andliquid cooling means for cooling said support.
 9. Apparatus according toclaim 6 wherein said cathode is surrounded by a heat insulating sleeveadapted to serve as a flow channel for the carrier gas in the area inwhich said anode is surrounded by carrier gas transporting particles ofceramic nuclear fuel.
 10. Apparatus according to claim 9 wherein saidheat-insulating sleeve is formed of ceramic.